Conjugate fibers composed of a crystalline poly-.alpha.-olefin, e.g., crystalline polypropylene, as a first component and a resin having a melting point lower than that of the first component as a second component, which can be fabricated easily into nonwoven fabrics by thermal adhesion at low temperatures, have been proposed, e.g., in Japanese Patent Publication Nos. 37097/77, 44773/79, 44774/79, 483/80 17807/80, 26203/80, and 26209/80. The low-temperature thermal adhesive resin as the second component in these conventional conjugate fibers include polyethylene, a copolymer of ethylene and vinyl acetate or a saponification product thereof, and the like.
Nonwoven fabrics obtained from these conjugate fibers have been chiefly utilized for the purpose of retaining water or other solutions in their voids, and the second component resin used is primarily intended for a drop in temperature for the thermal adhesion.
On the other hand, separators of sealed type lead accumulators, to which the nonwoven fabric made from the conjugate fibers of the present invention is chiefly applicable, have been prepared from a glass mat comprising fine glass fibers or nonwoven fabrics made of a single component, e.g., polyethylene, polypropylene, etc.
However, an ordinary glass mat undergoes shrinkage during use to lose intimate contact with grids, resulting in deterioration of performance as an accumulator. In addition, since the shrinked glass mat does not serve to retain an electrolyte liquor any more, the solution is released therefrom and descends therethrough, which leads to a reduction in discharge capacity. Therefore, it has been required to use an expensive mat composed of superfine glass fibers having fine voids.
Further, nonwoven fabrics comprising polyethylene, polypropylene, etc. are inferior in wettability by an electrolyte liquor due to their water repellency and, therefore, have poor retention of an electrolyte liquor. As a result, the electrolyte liquor is released therefrom and descends therethrough during use.